Remember that less text is more effective than more text. Your ad will appear on the HOME screen and on the interior sidebar. The sidebar ad is considerably smaller than the home page, so let that guide you on how much text to provide. Here is a sample of the sidebar ads: http://sanfranciscobookreview.com/product/child-decoded-unlocking-complex-issues-in-your-childs-learning-behavior-or-attention/
If someone can't read all the text you're providing, your ad will not benefit you.